DC/DC converters produce an output voltage at a different voltage level than an input voltage to the DC/DC converter. For example, DC/DC converters are commonly used to increase a DC input voltage to a higher output voltage or to decrease a DC input voltage to a lower output voltage. In addition, DC/DC converters provide electrical isolation and power bus regulation. DC/DC converters are employed in a variety of applications, including power supplies for personal computers, office equipment, spacecraft power systems, laptop computers, telecommunications equipment, and DC motor drives.
The input to a DC/DC converter is typically an unregulated DC voltage. The DC/DC converter produces a regulated output voltage that has a magnitude and/or a polarity that differs from the input voltage. Typical DC/DC converters employ switching devices, such as MOSFETs, IGBTs, BJTs and thyristors, to regulate and convert the input voltage. A controller controls the switching frequency and sequence of the switching devices to produce a desired output voltage. For example, the controller may implement a pulse width modulation (PWM) approach to vary the duty cycle of switching devices. With PWM, the switching frequency is constant and the duty cycle varies with load and voltage requirements.
DC/DC converters typically include a transformer that isolates the converter input and output. The transformer reduces the stress on the switching devices and improves the efficiency of the switching devices. Conventional PWM converters turn off the switching devices when current is flowing through them, which is commonly referred to as hard switching. When hard switching is used at high frequencies, relatively high switching losses occur. Switching losses are especially pronounced in high power, high voltage applications where hard switching is utilized.
To reduce switching losses, DC/DC converters implement either zero-current switching (ZCS) or zero-voltage switching (ZVS), which are commonly referred to as soft switching. In devices using ZCS, the switching devices are turned off when there is zero current flowing through the switching devices. In devices using ZVS, the switching devices are turned on when there is no voltage across the switching devices. Neither of these two distinct approaches strike an optimum balance between switching and conduction losses.